Biological Control

Biological control
refers to the control of algae or aquatic plants by increasing the abundance
of herbivores. Although relatively new as a scientifically supported management
option, biological control has, in fact, been practiced for centuries.
Several ecosystems have been irreparably changed due to the purposeful
release of one organism or another. It is now accepted among scientists
that ecosystems are very complex systems and introduction of a species
brings several (often-unanticipated) changes. For algae and plant
control in ponds, there exist a few biological control techniques, including
introduction of herbivorous fish (e.g., sterile grass carp, tilapia), and
encouraging zooplankton populations. Placement of decaying barley straw
in the pond is believed to curb algal growth by stimulating the release
of inhibitory chemicals by bacteria. As control, if any, is likely due
to chemicals released by the decomposing straw, it is discussed separately
in the chemical treatments section.

1. Grass Carp (White Amur)

Grass carp (Ctenopharyngodon idella ) a fish this size could be
safely stocked into a pond with adult largemouth bass ( Micropterus
salmoides)

The grass carp (Ctenopharyngodon idella
), also called White Amur, is native to China where it normally inhabits
rivers. This species does quite well, however, in a pond setting. Only
sterile (triploid) fish may be legally released in Pennsylvania. With three
sets of chromosomes instead of the usual two, triploid fish are unable
to reproduce should they escape into the wild. A permit from the PA Fish
and Boat Commission is needed to sell, transport or possess Grass Carp.
A fisheries scientist will review the application and make a stocking recommendation
based on the information provided.

As grass carp have been introduced
in many localities for plant and algae control under a variety of conditions,
there naturally exist both positive and negative anecdotal reports concerning
them. Because each pond/lake is unique, a detailed knowledge of their biology
should form the basis for deciding whether to introduce them into a pond
or lake. Check laws in your state before purchasing or introducing any
fish for aquatic weed control. There are several considerations to keep
in mind when determining whether grass carp are appropriate for an introduction:

Grass carp are river fish in their native habitat and will readily seek
out flowing water; grates must be installed to prevent fish from leaving
ponds where they are introduced.

1. Grass carp will consume any plant that is available
to them, in the order of THEIR preference. The fish may not have a preference
for the plant that is least desirable from the pondowner's perspective,
and their presence may simply shift the balance of a plant community to
more unpalatable types. Grass Carp may preferentially remove plants that
are needed as habitat for desirable fish species in the pond. Grass Carp
will eat filamentous algae, but it is not a preferred food item.If
there are plenty of submersed plants that the fish find palatable, they
may ignore filamentous algae completely.

2. Grass carp have extraordinarily inefficient digestive systems
and only assimilate a small amount of what they take in. Material that
the fish excrete returns to the sediments where it can contribute to further
plant or algal growth.3. Grass carp are very shy fish; once introduced, removing them
without draining the pond or harming other fish is difficult if not impossible.4. Stocking densities are difficult to calculate without sufficient
data. If the fish are over-stocked they can remove all the vegetation in
the pond; if understocked they will fail to provide adequate control.5. Grass carp are tolerant of highly turbid and almost anoxic (0.5mg/l
oxygen) water1. They may be able to survive in water that is
not healthy for other fish. The bottom line is that introduced grass
carp should not be expected to be a cure-all. Combining other measures
such as herbicides and colorants with grass carp may provide better control
than any measure used alone.

2. Other fish

Other fish that have been introduced for control
of vegetation in some localities with considerably less success include
true algae-eating fish such as tilapia and gizzard shad. The gill rakers
of these fish are spaced narrowly enough to facilitate removal of planktonic
algae from the water column. Neither species is appropriate for release
in Pennsylvania ponds. Tilapia (collective name for fish in the
genera Sarotherodon, Oreochromis and Tilapia ) are
tropical fish, common in aquariums. Though the blue tilapia (Oreochromis
aureus ) has survived in Florida and Texas, most other states are too
cold for this fish to survive. It has been suggested that these fish become
sluggish before dying, allowing them to be consumed by gamefish such as
largemouth bass. If tilapia were to be stocked for algae control, they
would need to be restocked yearly. Releasing non-native fish into PA waters
may be illegal, even if the pond is privately owned and is not connected
to other bodies of water. Always check with the PA Fish and Boat Commission
before stocking your pond. Gizzard shad (Dorosoma cepedianum
) are also known to eat phytoplankton by filtering water as they swim.
Unfortunately, they also filter out zooplankton effectively, much more
so than juvenile bluegills. Gizzard shad may also grow too large to serve
as forage fish for Bass, and muddy the pond bottom when feeding. As such,
negative impacts on bluegill/largemouth Bass populations in the pond heavily
outweigh their usefulness as an algae control agent.

3. Zooplankton

The
zooplankton (a collective term for microscopic fauna found living in the
water column), consists chiefly of protozoa, microcrustaceans and rotifers.
Three types of microcrustaceans are depicted at left. Zooplankton grazers
consume a wide range of algae, while some species consume other zooplankton.
Larger species of zooplankton are important in the diets of many fish species,
especially as fry. If fish predation on zooplankton is reduced, they may
contribute to a reduction of phytoplankton and a desirable change in water
clarity. Efforts to increase the role of zooplankton
grazers as effective biological controls of algal growth often seek to
reduce fish predation by creating refugia. Deepening the pond may be effective.
Limnological studies have shown that some types of motile zooplankton exhibit
a migration pattern vertically through the water column, coming up to feed
at night when visibility is poor and returning to the depths during the
day. Increased control of algae by zooplankton may thus result from dredging.